Mass spectrograph for imaging low-energy
neutral atoms
Arthur G. Ghielmetti
Edward G. Shelley
Stephen A. Fuselier
Lockheed Palo Alto Research Laboratory
D/91-20, B255
3251 Hanover Street
Palo Alto, California 94304
Peter Wurz
Peter Bochsler
University of Bern
Physikalisches lnstitut
3012 Bern, Switzerland
Federico A. Herrero
Mark F. Smith
NASA Goddard Space Flight Center
Laboratory for Extraterrestrial Physics
Greenbelt, Maryland 20771-0001
Abstract. We describe an instrument concept for measuring low-energy
neutral H and 0 atoms with kinetic energies ranging from about 10 eV
to several hundred. The instrument makes use of a low work function
surface to convert neutral atoms to negative ions. These ions are then
accelerated away from the surface and brought to an intermediate focus
by a large aperture lens. After deflection in a spherical electrostatic ana-
lyzer, the ions are postaccelerated to —25-keV final energy into a
carbon-foil time-of-flight mass analyzer. Mass resolution is adequate to
resolve H, D, He, and 0. Energy and azimuth angle information is obtained by means of position imaging the secondary electrons produced
at the carbon foil. A large geometric factor combined with simultaneous
angle-energy-mass imaging that eliminates the need for duty cycles provide the necessary high sensitivity. From a spinning spacecraft this instrument is capable of producing a 2-D map of low-energy neutral atom
fluxes.
Subject terms: magnetospheric imagery; atmospheric remote sensing; neutral
atom imaging; ion optics; time-of-flight spectrometry; surface conversion.
Optical Engineering 33(2), 362—370 (February 1994).
Thomas S. Stephen
University of Denver
Physics Department
Denver, Colorado 80208
1 Introduction
The objective of the inner magnetospheric imager mission
presently under consideration by NASA is to obtain global
maps ofthe distributions ofplasmas in the Earth's ionosphere
and magnetosphere. Fast neutral particles created from ions
that undergo charge exchange on collision with neutral gas
atoms of the upper ionosphere and geocorona offer the potential for imaging the original charged particle distributions.
These neutrals are well suited for remote sensing because
they are unaffected by the magnetic field and, thus, follow
ballistic paths. Neutral atom imaging is a relatively new tech-
nique"2 that requires significant improvements in detector
and analyzer methods to make it suitable for sensing the
typically low fluxes of neutral atoms. The High-Latitude Ion
Transport and Energetics (HI-LITE) Explorer3 proposed to
Paper Ml-029 received April 25, 1993; revised manuscript received Aug. 25, 1993;
accepted for publication Aug. 30. 1993.
1994 Society of Photo-Optical Instrumentation Engineers.
0091 -3286/93/$6.00.
investigate the global ion outflow from the high-latitude terrestrial ionosphere includes this type of novel neutral atom
imaging instrument.
Conventional techniques for measuring neutral particle
fluxes rely either on direct detection via the energy deposition
in solid state detectors, or on ionization and subsequent analysis in charged particle instruments. The former technique
has been successfully applied for imaging energetic (> 20keV) atoms originating in the terrestrial ring current.2 However, these techniques cannot be applied efficiently to much
lower energies.4 Because low-energy neutral atom fluxes in
the planetary ionospheres and magnetospheres and in inter-
planetary space are generally many orders of magnitude
lower than charged particle fluxes,' a highly efficient ionization process is called for. Neither electron nor field ionization with their low overall yield, nor photon ionization with
its impractical demands on spacecraft resources (size, weight,
and power), meet these requirements.
Transmission ionization employing thin carbon foils
(C foils) provides reasonably high yields (—10%) down to
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MASS SPECTROGRAPH FOR IMAGING LOW-ENERGY NEUTRAL ATOMS
energies as low as 1 keV/nucleon.57 Using ultrathin C foils
in combination with highly sensitive analyzers, McComas et
al.8 have reported good efficiencies for negative 0 production
at energies as low as 1 keV. However, at energies below
approximately 0.5 keV/nucleon, the yield of secondary
electrons9 and of ions5'6 decreases rapidly. As a result, the
very low energy regime of neutral particle fluxes remains
thus far largely inaccessible to in situ measurements from
spacecraft.
In recent years, new surface ionization techniques have
been developed for laboratory applications that are capable
of providing high ionization yields at energies below
1 keV.'°'2 These techniques make use oflow work function
surfaces for converting neutral particles to ions during surface
impact and reflection. A similar system that relies on in-flight
conversion surface regeneration is presently in operation on
the Ulysses solar probe.'3 Surface ionization introduces new
demands on design that differ from conventional spaceflight
instruments and requires the development of new analyzer
elements with matched ion optical properties.
In this paper we describe the concepts for an instrument
capable of imaging low-energy neutral H, 0, and He atoms.
Such an instrument would be suitable for imaging neutral
atom fluxes from such diverse source regions as the cleft ion
fountain and the interstellar neutral wind. An instrument
based on the principles described here was proposed for flight
on a NASA small explorer mission to image the chargeexchanged H and 0 neutrals from the high-latitude terrestrial
ionosphere. Alternate concepts that combine conversion ionization techniques with a spectrograph have been described
by Herrero and Smith'4 and Gruntman.'5
2 Instrument Description
2.1 Overview
The imaging low-energy neutral analyzer (ILENA) proposed
here combines state-of-the-art laboratory technology with
flight-proven space plasma analyzer technology. A schematic
cross section of the sensor in a plane containing the axis of
symmetry is shown in Fig. 1 . The principal elements of the
ILENA are an entrance collimation system, a conversion unit,
an extraction lens, an electrostatic analyzer, and a C-foil timeof-flight (TOF) mass analyzer with position sensing. Neutral
and charged particles enter the sensor via the external aperture
B 1 and are collimated in angle and area by the S 1 entrance
slit. An electrostatic deflector removes all ions with energies
< 100 keV, while a broom magnet deflects all electrons with
energies <200 keV. The remaining neutral particles proceed
to the conversion plate (C) where they impact at an oblique
angle. At the conversion plate, some of the neutrals undergo
charge exchange and become negative ions. These ions are
accelerated away from the converter surface and are focused
by a wide-aperture low-aberration lens (L) in the 52 slit plane.
The conical slit 52 is set to transmit ions with initial energies
within a passband of -10< E < 300 eV. Transmitted ions
are subsequently imaged by a spherical electrostatic analyzer
(EA) geometrically configured to be focusing in elevation
angle in the image plane of the C foil. Before entering the
TOF section, ions are postaccelerated to about 25 keV. On
striking the C foil placed in the focal plane of the EA, the
negative ions produce secondary electrons that provide the
start pulse as well as the azimuth-radial position information.
The transmitted particles (ions and neutrals) proceed to the
stop microchannel plate (MCP).
The entire instrument is rotationally symmetric about a
vertical axis collocated with the 51 entrance slit. Ions that
enter the analyzer through the Si slit maintain their initial
velocity direction except for nonspecular reflection at the
conversion surface. These effects are minimized through a
special ion optics design of the acceleration lens system. As
a result, a direct correlation between the incident azimuth
direction and the particles position in the plane of the C foil
is achieved that enables one to deduce the original velocity
direction of the neutral atom. In a similar manner, energy
information is extracted from the radial impact position.
2.2 Collimator
All particles enter the instrument through aperture B 1 and
pass through a baffle system that prevents forward scattering
ofphotons and particles through the use of serrated blackened
(CuS) surfaces. A pair of horizontal deflection plates sweep
-
out charged particles with energies per charge less than
100 keV/e from the converter surface, and a small broom
magnet deflects electrons with energies <200 keV. The required deflection plate voltages are —5 kV. The collimator
is fan shaped to provide the desired wide azimuth acceptance,
while the elevation angle acceptance is defined by the heights
of the Bi and 51 slits. A gas-tight shutter located behind 51
can be closed on command to protect the converter surface
during launch and during the perigee portion of a low-earth
orbit.
2.3 Conversion
Neutral atom fluxes are generally orders of magnitude lower
than charged particle fluxes. In the case of the cleft ion foun-
tam, for example, the expected peak fluxes of less than
— 1 00 eV neutral atoms are between — 1 O and — iO I
(cm2 sr s) at a distance of several thousand kilometers.'6
Hence, a highly efficient ionization mechanism is required
to obtain images of this source region on reasonably short
time scales. The approach taken here is to use a low work
function converter surface for converting atoms with positive
electron affinity to negative ions. This type of charge exchange process works well for H and 0 atoms that have high
electron affinities (0.75 and 1.46 eV). Particularly low work
functions are obtained with monocristalline tungsten W(1 10)
substrates coated with a thin layer (—0.6 monolayer) of Cs
(see Ref. 17). The Cs overlayer reduces the work function
(to — 1.5 eV) and, in combination with the high-electron density of the W substrate, facilitates the electron transfer to the
2 Conversion
reflected
studies, which have primarily
focused on H and D because of their importance to fusion
machines, have provided H- yields of up to 67% under ideal
laboratory conditions. In contrast, only a few experimental
measurements exist for the conversion efficiencies of 0 and
He atoms. These have indicated high negative ion yields of
66% for 0 (see Ref. 1 8) and 14% for He (see Ref. 19), respectively. Because optimal control of the thickness of the
Cs layer is probably more difficult to achieve in flight than
in a laboratory environment, we prudently assume a somewhat lower conversion efficiency because of a thicker Cs
coverage (Table 1). The results of model calculations for the
conversion efficiency from a CsIW(1 10) surface as a function
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GHIELMETTI et al.
Fig. 1 Schematic drawing of the ILENA instrument with principal components. The entrance collimation system with elevation angle acceptance slits Bi and Si, ion and electron deflectors 1-DEF and
E-DEF, conversion surface C, cesium dispensor D, secondary electron guiding magnets Mi and M2,
extraction lens L, energy limiting slit S2, spherical electrostatic analyzer EA, and the TOF mass analyzer MA.
Table I LENA instrument summary for the HI-LITE mission.
Energy Range
: 10 eV to 300 eV
Resolution
- 5 steps
Mass Species
: H, 0, He
Field of View
Azimuth
Elevation
>,
0
a)
0
: 8°
90°
a)
C
0
Cl)
a)
>
C
Angle Resolution
Azimuth
: 100
Elevation
100
Conversion Efficiency
0.1
TOF efficiency (c0.e&.'rä)
: 0.4
Geometric Factor
: 2 x 10-1 cm2-sr
of impact angle are shown in Fig. 2 for the case of an optimal
Cs coverage. A more detailed description of the derivation
can be found in Ref. 20.
The conversion unit consists ofindividual azimuthal facets
each aligned on a conical surface centered about the axis of
rotational symmetry. The width of each facet is primarily
driven by the azimuth angle resolution requirements. For the
HI-LITE instrument,3 nine facets, each spanning 10 deg in
azimuth, were considered. The optimal orientation of the
converter surface represents a trade-off between the sensitivity and energy resolution requirements and the size and
00
22
44
66
88
angle from surface normal (deg)
Fig. 2 Conversion efficiency for H reflected from a W(i iO) surface
covered with 0.6 monolayer of Cs as a function of impact angle.
weight constraints imposed by the mission. The impact angle
of —65 deg from the surface normal considered here provides
a high conversion efficiency (Fig. 3) and high geometric fac-
tor with an acceptable converter size.20 As discussed in
Sec. 2.4, to achieve good energy resolution requires a narrow
angular spread in the reflected ions and, thus, near-specular
reflection. For an impact angle of —80 deg, the expected fullwidth at half-maximum angular spread of the reflected ions
is < 10 deg (see Ref. 10) while the conversion efficiency
remains high (Fig. 2). However, at this incidence angle the
geometric factor is reduced by a factor of 2.5 compared to
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MASS SPECTROGRAPH FOR IMAGING LOW-ENERGY NEUTRAL ATOMS
30
an impact angle of 65 deg. The final choice for the impact
angle is mission dependent and will be a compromise between
the above parameters.
The converter unit includes a Cs dispenser (D) and a heater
for baking the surface to remove undesired adsorbate contaminations. This allows for periodically reconditioning the
converter surface during flight when needed. As discussed
above, the ideal Cs-layer thickness is somewhat less than a
monolayer. To verify the condition of the surface, we must
measure its work function, because this is the determining
parameter for the conversion efficiency. We accomplish this
by illuminating individual azimuth segments with light from
several (three) laser diodes at different wavelengths. The
wavelengths of the laser diodes are strategically selected to
obtain a relative measure of the work function at key points
Photoelecnear the minimum of the work function
trons emitted from the converter surface are collected on a
positively biased anode with the aid of a guiding magnet
(M 1-M2). This current provides a measure for the conversion
efficiency because the photon and negative ion production
rates are both governed by the work function of the surface.
During this operation the entrance slit shutter is closed to
prevent interference by external EUV light.
The required regeneration frequency will depend on the
rate of deterioration of the surface. Special care must be taken
to prevent contamination of the converter surface at launch
and from low-altitude exospheric gases. Diffusion of neutral
spacecraft outgassing products into the sensor can be kept
negligible compared to the internal sensor pressure through
proper spacecraft mounting techniques.2' Generally, if the
gas load to the converter surface is kept to less than one
monolayer (-- 1O' atoms/cm2), the effects on conversion efficiency are expected to be small. For example, at altitudes
of — 1 Re the expected average ram flux of H, the dominant
constituent, is < 108 atoms/(cm2 s), thus requiring more than
100 days for a monolayer of H to form (assuming perfect
sticking). The entrance slit shutter will be closed during
launch and during the low-altitude portion of a satellite orbit
to protect the conversion surface. In the case of the proposed
HI-LITE mission, we conservatively estimate that regeneration of the surface will not be necessary more frequently
than every few days.
Because the primary gas load to the converter stems from
the instrument internal residual gases, a clean ultra-high vacuum (UHV) must be maintained throughout the operational
period of the sensor. For example, at an internal residual gas
pressure of iO-9 Ton, and assuming a sticking probability of
unity, a single monolayer would form in about 40 mm. However, the surface degradation resulting from adsorbed gases
depends to a large extent on the chemical properties of the
molecules or atoms. Thus, materials that outgas electronegative atoms and molecules, such as F and Cl, must be
avoided. Hence, only metal and ceramic materials will be
used in interior structures and surfaces. Standard space instrumentation techniques for maintaining a high degree of
chemical cleanliness within the sensor will be used. These
include thorough cleaning and vacuum bake out of individual
components, assembly under dry nitrogen clean-hood conditions, and vacuum bake out of the entire instrument after
assembly with a subsequent dry-N2 purge until launch. The
converter substrate itself will be subjected to a hightemperature bake out prior to installment into the sensor. A
25
>
0
20
a)
0
a)
15
0
10
a)
>
0
0
5
10
102
energy (eV)
Fig. 3 Calculated conversion efficiency for hydrogen at an impact
angle of 65 deg reflected from a W(11O) surface covered with 0.6
monolayer or a thick layer (1 monolayer) of Cs taking into account
the ionization efficiency and reflection probability.
separate bypass venting port will provide a tenfold-enhanced
pumping speed to outer space. With these precautions, we
expect that the final steady state pressure within the sensor
can be kept significantly below i09 Torr throughout the operational phase of the orbit.
2.4 Extraction Optics
After acquiring a charge, the negative ions are accelerated
away from the surface by a wide aperture lens system (L).
The potential distribution of the lens (Fig. 4) was designed
to efficiently collect the ions produced on C and to focus
them in the plane of the 52 slit. Equipotentials are parabolically shaped over much of the interior region to minimize
spherical aberrations. Figure 5 illustrates the energy dispersive properties of the acceleration lens; reflected ions are
focused toward larger radii with increasing energy. To transmit the desired energy range from 10 to about 300 eV, the
collimating slit 52 needs to be — 1 cm wide. Angle scattering
about the specular reflection direction at the converter broad-
ens the focal spot. As seen from Fig. 5, the broadening for
lO-deg scatter is much less than the energy dispersion.
Hence, it is possible, in principle, to extract moderate energy
information from the radial position, provided that angle scat-
tering and energy straggling are sufficiently small. As discussed earlier, this requires near-grazing incidence impact
angles.
To minimize the effects of nonspecular reflection on azimuth resolution (out of plane of Fig. 1), we must apply a
high extraction field near the surface and have a high ratio
of final-to-initial energy. A lens extraction voltage of about
10 kV represents a good compromise between the requirements for energy dispersion and azimuth angle resolution.
With this voltage, the beam spread in the azimuthal direction
is sufficiently small to allow for —5-deg azimuth resolution.
2.5 Energy Analyzer
Before entering the energy analyzer (EA) all ions are further
accelerated to —20 kV between the object slit S2 and the
Herzog field limiting plate Hi. The spherical analyzer is
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GHIELMETTI et al.
C-Foil
Axis
S2
H2
0
45
90
Fig. 4 Equipotential lines in extraction lens between converter surface C and exit slit S2.
S2
Fig. 6 Ray tracing of ions in the spherical electrostatic analyzer originating at the object slit S2 with angle and energy dispersion defined
by the extraction lens. Geometric parameters are 62-mm center radius, 155-deg sector angle, and 24-mm plate separation.
2.6 Time-of-Flight Mass Analyzer
Following deflection in the EA, the ions are postaccelerated
to the C foil at a potential of + 25 kV. The mass analyzer is
a TOF device using the conventional C-foil technique.22 The
actual design is similar to the CODIF and TEAMS
S2
L
Fig. 5 Trajectories of ions originating at the converter surface with
energies of 10 and 100 eV, respectively. Reflection angles are
10 deg from the specular direction (65 deg). Total acceleration potential to S2 is 10 kV.
geometrically configured to image the entire object slit S2
onto the C foil of the TOF mass analyzer. Because the image
is inverted (magnification --- 1), the more energetic ions are
mapped to smaller radii. These elevation angle focusing and
radial dispersion properties are illustrated in Fig. 6. To
achieve this unity transmission requires an unusually wide
plate separation for the spherical analyzer. Although this condition does not significantly affect the imaging properties of
the analyzer, it does lead to a small analyzer constant (—1.2)
and, consequently, high plate-to-plate voltages. However,
since these voltages need not be scanned they can be directly
derived via voltage dividers from the static TOF acceleration
voltage. The analyzer further provides for the desired additional UV-light filtering by introducing multiple surface reflections and solid angle attenuation.
spectrometer3 on the CLUSTER and FAST missions. The
principal components ofthe TOF mass analyzerare the C foil,
a drift path for the ions and neutral particles, a secondary
electron extraction optics, and two MCP detector units. Secondary electrons produced on the back side of the C foils are
accelerated and focused onto MCP1 (Fig. 1), producing both
the start pulse for the TOF and providing radial and/or position information. The signal produced at MCP2 by the primary particle provides the stop pulse. The azimuthal position
can be provided by either MCP1 or MCP2. Both the start
and stop signals are obtained from low (50%) transmission
grids placed behind the respective MCPs. The anode behind
the MCP1 grid is divided into —5 radial segments providing
moderate energy resolution. Similarly, the anode behind the
MCP2 grid is divided into nine sectors providing lO-deg
azimuth angle resolution. The position and TOF electronics
are contained within the high-voltage (HV) bubble. The encoded information is transmitted across the HV interface via
fiber optics.
Because this instrument is sensitive to atoms with positive
electron affinities, the TOF mass analyzer needs only to be
able to separate H, D, He, and 0 in a single charge state.
Figure 7 shows a simulated mass spectrum for a 2 g/cm2
C foil and for the typical abundances expected in the upper
ionosphere at — 1000-km altitude. The expected mass resolution of the TOF mass analyzer for all ions of interest is
given in Table 2.
3 Discussion
Early spaceflight instruments performed differential measurements of ion distributions and required scanning in mul-
tidimensional parameter space for obtaining distribution
functions. While most newer instruments are imaging spectrographs,8'23'24 they still rely on plate voltage scans for covering the energy range. In the ILENA instrument we have
carried the development one step further by simultaneously
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MASS SPECTROGRAPH FOR IMAGING LOW-ENERGY NEUTRAL ATOMS
mean life25 of > -- 10 jis, a large fraction (> 97%) of the
converted He is expected to anive in the TOF section because
its transit time from the converter to the C foil is only
.u)
C
a)
C
C
0)
C')
10
mass [amu]
Fig. 7 Simulated mass spectrum for the TOF mass analyzer using
ion abundances occurring at an altitude of 1000 km. The ion abundances and the mass resolution used for this simulation are given
in Table 2.
Table 2 Mass resolution of the TOF mass analyzer at an ion energy
of 25 keV for a C-foil thickness of approximately 2 p.g/cm2. Relative
abundances at an altitude of 1000 km are given. Resolution data is
adapted from measurements reported in Ref. 22.
species
abundance
[relative to H]
energy per
nucleon
rn/Lrn
[keV/nuc]
1H
25
0.028
17.8
1.8 i04
12.5
0.039
12.8
3He
5 10-5
8.33
0.05
10 0
4He
0.1
6.25
0.06
8.3
160
1
1.56
0.14
37
2D
1
imaging over the three parameters azimuth angle, energy,
and mass. This triple spectrographic imaging eliminates the
need for a duty cycle, thus increasing sensitivity by the ratio
of cycle period to accumulation time per cycle step. Cornpared to conventional energy scanning analyzers (16 energy
steps), the ILENA provides a '—20 times higher sensitivity.
The absence of fast high-voltage scanning provides the added
benefit of simplified and lighter HV supply design.
The requirements on mass resolution on an instrument
designed for imaging the high-latitude ionospheric acceleration regions are rather modest when compared with proven
designs presently under development for spaceflight.23 At the
very least, a resolution of H and 0 at the 0. 1% level is fully
adequate for this type of investigation. For investigations of
the interstellar neutral wind, we would ideally like to be able
to additionally resolve D, He3 and He4 at their isotopic abundance levels (see Table 2). Despite its low-electron affinity
(0 eV), the ionization efficiency reported for He is remarkably
high.'9 Although the He ion is unstable as it decays with a
—0.25 ps. While the proposed design easily separates D and
He, its resolution is marginal for He3 and He4. Further irnprovernents in mass resolution can be achieved with thinner
C foils and/or higher postacceleration voltages.
The rotational symmetry of this design makes it possible
to obtain a wide field of view (FOV) in the azimuth direction
coupled with simultaneous imaging of the incident azimuth
angles. Although the azimuth acceptance ofthe present optics
is geometrically limited to — 160 deg due to the flat FOV,
such an instrument would nevertheless be able to cover more
than 90% of the full 4-'rr solid angle from a spinning spacecraft. Using a conical entrance FOV would permit us coyerage twice per spin. For source regions of spatially limited
extent, such as the auroral acceleration region, a 2-'rr instantaneous FOV provides no further benefits in geometric factor.
The azimuth resolution that can be achieved with this type
of instrument is intrinsically limited by the width of the entrance slit S 1 , the radius of the conversion surface, and the
magnitude ofthe lens extraction voltage. For a 1 -cm2 entrance
slit area and a 10-cm radius, a resolution of ---5 deg is feasible
in principle.
The ability to detect very low fluxes of neutral atoms relies
on a low background from all sources. Although the TOF
coincidence technique used here is inherently less sensitive
to background, the large fluxes of EUV photons from the
Sun, geocorona, and the Earth' s limb would cause a serious
background problem if not suppressed. In the proposed design, the required attenuation is achieved by a combination
of efficient light traps,26 multiple and diffuse surface reflections (three), and low reflectivity ''black' ' surface coatings2'
at critical places. The photon background rate at the detectors
due to geocoronal EUV is estimated to be < lOs ' . At this
rate, the probability for chance coincidences from photons
producing a secondary electron at the C foil and at the stop
detector within the flight time window is negligibly small
(<0.01s '). Besides the photon background, the two other
major potential sources of background are photoelectrons
from the converter surface and negative ions produced by
dissociative attachment of photoelectrons to residual gas molecules. Due to the low work function ofthe converter surface,
photoelectrons will be produced in great quantities. These
electrons would generate unacceptably high chance coincidence rates if allowed to reach the TOF system, even though
individually they could be discriminated against due to their
short flight times. To divert and trap these electrons, a weak
magnetic field is applied between the pole faces of Ml and
M2 (Fig. 1). As discussed in Sec. 2.3, the same trap serves
as a collector for the photoelectrons during converter surface
regeneration. The negative ion background is minimized by
maintaining a low-residual gas pressure inside the instrument.
Furthermore, by quickly accelerating the photoelectrons
away from the sensitive region, the probability for electron
attachment is further reduced because the cross section for
this process is inversely proportional to the kinetic energy.
For example, at a partial H20 pressure of 1 X 10 Ton,
the negative ion production rate is estimated to be about
1 5 — . Because these ions have very low energies
(<< 1 eV), they are tightly focused onto the low-radius part
of the 52 collimator where they are efficiently eliminated.
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GHIELMETTI et al.
Negative ions produced from the ambient neutral atmosphere
in the ram direction have low energies and are blocked by
the S2 collimator in the same way.
Sputtered negative ions, originating from energetic ion
bombardment on the converter surface, also represent a potential source of contamination. Of primary concern are adsorbed gases because W or Cs ions have very low transmis-
sion efficiencies in the TOF section and can easily be
recognized by their unique heavy mass. Because energetic
ions are abundant in the Earth's orbit, it is essential to reduce
their fluxes. In the present ILENA design, ions with energies
up to 100 keV/e are rejected by an electrostatic deflector in
the entrance collimation system.
A critical element of the ILENA instrument concept is the
neutral to ion conversion technique. As discussed in Sec. 2.3,
the optimum conversion efficiency is attained with a —0.6
monolayer of Cs on a W(1 10) substrate, but reasonably high
efficiencies are also obtained with somewhat thicker layers.
The problem of maintaining a near-optimum layer thickness
is facilitated because thicker layers evaporate quickly due to
the high vapor pressure (106 Torr) of Cs at room temperature, leaving behind a stable monolayer that has a vapor
After reconpressure several orders of magnitude
ditioning the surface for an estimated 10 to 100 times during
a mission, the average contamination with Cs on internal
surfaces of the sensor, other than the converter itself, is expected to be less than one monolayer. Because the vapor
pressure of a Cs monolayer is very low, migration of Cs
between different parts of the instrument is minimal. Nevertheless, special design precautions are taken to guard all internal high-voltage insulators (for example, individual guard
baffles). No deterioration of the MCP detectors, which are
well shielded behind the C foils, is expected at the estimated
Cs evaporation levels.
While cesiated conversion surfaces have been thoroughly
investigated in the laboratory environment, to our knowledge
they have thus far not been applied to space plasma instrumentation. Our goal in our ongoing research effort is to de-
velop this novel technology for use on spacecraft instrumentation. Because surface cleanliness is of particular
concern for Cs-W surfaces, other candidate surfaces that pro-
vide low work functions but are less susceptible to degradation, such as, for example,2728 Cs-O and Ba, are being
investigated in parallel for this instrument.
4 Conclusions
The energy range (10 to 300 eV) covered by the proposed
new instrument is presently inaccessible to conventional
C-foil transmission techniques proposed for measuring the
more energetic > 1 keV/nucleon neutral atom (ENA) fluxes.
This very low energy regime carries some of the highest
neutral atom fluxes found in the magnetosphere. For example,
the luminosity of the brightest areas of the high-latitude acceleration region'6 substantially exceeds that of the ENA
fluxes from the ring current.2 Surface ionization is thus a new
technology that ideally complements the energy range coyered by the foil transmission technique, and that offers great
potential benefits to future ionospheric, magnetospheric,
planetary, and astrophysical investigations.'5
Acknowledgments
We acknowledge the technical and engineering support pro-
vided by T. Sanders. This research was supported by Lockheed Independent Research and by the Swiss National Science Foundation.
References
1_
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We have developed the concept of a new type of mass spec-
trograph that is specifically designed for imaging low-energy
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covers the energy range from about 10 to 300 eV, and the
mass range from 1 to 20 amu with mass resolution sufficient
to separate the major masses of interest H, D, He, and 0.
The instrument's large geometric factor and wide FOV combined with high conversion, transmission, and detection efficiencies (Table 1) and 100% duty cycle give it the necessary
high sensitivity for imaging the low-energy outflow from the
high-latitude ionospheric acceleration regions. Based on the
model calculations reported in Ref. 20, we estimate count
rates of —1 to 10 Hz in a lO-deg azimuth pixel for the HILITE orbit, thus providing a well-defined image within about
5 mm.
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14.
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MASS SPECTROGRAPH FOR IMAGING LOW-ENERGY NEUTRAL ATOMS
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Stephen A. Fuselier is a research scientist at Lockheed Palo Alto Research Laboratory. His major areas of interest are solar wind magnetospheric coupling and bow
shock/fore shock phenomena. He received
a BS in 1981 from the University of Southem California and a MS in 1983 and a PhD
in 1984 from the University of Iowa, all in
physics. He held a two-year postdoctoral
position at Los Alamos National Laboratory. He joined Lockheed in 1986 as an
associate research scientist and was promoted to research scientist
in 1989. He is the author of more than 60 publications in journals
and conference proceedings. Fuselier is a member of the American
Geophysical Union and has served on the Space Physics and
Aeronomy Education Committee since 1990.
M. Ertl, E. Künneth, C. W. Carlson, D. W. Curtis, R. P. Lin, J. P.
McFadden, J. Croyle, V. Formisano, M. Di Lellis, R. Bruno, M. B.
Bavassano-Cattaneo, B. Baldetti, 0. Chionchio, E. G. Shelley, A. G.
Peter Wurz obtained an engineering degree in electronics and attended the Technical University of Vienna where he received a MS and PhD in physics. From
1 990 to 1 992 he performed postdoctoral
work at Argonne National Laboratory, Chi-
Ghielmetti, W. Lennartsson, A. Korth, H. Rosenbauer, I. Szemerey, R.
Lundin, S. Olsen, G. K. Parks, M. McCarthy, and H. Balsiger, "The
ion spectrometry experiment in cluster: mission, payload and supporting
activities," ESA SP 1159, 133—161 (1993).
24. E. G. Shelley, A. G. Ghielmetti, H. Balsiger, R. K. Black, J. A. Bowles,
R. P. Bowman, 0. Bratschi, J. L. Burch, C. W. Carlson, A. J. Coker,
J. F. Drake, J. Fischer, J. Geiss, A. Johnstone, D. L. Kloza, 0. W. Lennartsson, A. L. Magoncelli, G. Pashmann, W. K. Peterson, H. Rosenbauer, T. C. Sanders, M. Steinacher, D. M. Walton, B. A. Whalen, and
D. T. Young, ' 'Thetoroidal imaging mass-angle spectrograph (TIMAS)
for the polar mission,' ' Space Sci. Rev. (submitted for publication).
25. B. L. Donnally and G. Thoeming, ' 'Helium negative ions from matastable helium atoms,' ' Phys. Rev. 159, 87—80 (1967).
26. F. A. Herrero, ' 'Light trap cavity design using multiple reflections and
solid angle attenuation: applications to a spaceborne electron spectrom-
eter,"
cago, Illinois, in the field of molecular mass
spectroscopy of fullerenes, biomolecules,
and polymers. Since 1992 he has held a
position at the University of Berne, Switzerland, in the mass spectroscopy group.
He has published papers in the fields of ion sputtering, electronstimulated desorption, ion optics, physical chemistry, laser interaction with solids and particles, and extraterrestrial physics.
AppI. Opt. 31, 5331—5340 (1992).
M. Seidl, S. T. Melnychuk, S. W. Lee, and W. E. Can, ' 'Surface production of negative hydrogen ions by reflection of hydrogen atoms from
cesium oxide surfaces,' ' in Production and Neutralization of Negative
Ion Beams, A. Hershcovitch, Ed., American Institute of Physics Conference Proceedings 210, 30—39, New York (1990).
28. C. F. A. van Os, P. W. Amersfoort, and J. Los, ' 'Negative ion formation
at a barium surface exposed to an intense positive-hydrogen ion beam,"
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27.
Peter Bochsler is an associate professor
of physics at the University of Bern, Switzerland. His main research interests include
space physics, solar wind, and history of
the Sun and of the solar system. He graduated from the University of Bern with a
thesis on cosmic-ray-produced noble gas
isotopes in meteorites. His PhD thesis
dealt with 3H (tritium) in lunar soil. He
—
Arthur G. Ghielmetti is currently a staff
scientist at the Space Sciences Laboratory
at the Lockheed Palo Alto Research Laboratory. He has been a co-investigator on
7 NASA and ESA space research missions. He received his BA degree in physics from Cornell University and his MS and
PhD in physics from the University of Bern,
Switzerland, in 1974. His research interests include electron and ion spectrometry,
— sensors, space plasmas instrumentation,
and space plasma research. He is the author of more than 35 papers
on ion optics and space plasma research.
, c,s in terrestrial samples. Returning to the University of
Bern, he continued his research on terrestrial samples and, simultaneously, he participated in the development and calibration of the
ISEE-3 Ion Composition Instrument. After the successful launch of
ISEE-3, he took an active role in the data analysis and interpretation
with special emphasis on the abundances of minor elements in the
solar wind, the isotopic composition of solar wind helium, and the
dynamics of minor ions in the solar wind plasma. Presently, he is
involved as a member of an international team in the development
of the CELIAS experiment on the SOHO payload, and in collaboration with a team from the University of Maryland, in the development of the WIND-MASS and the WIND-SWICS experiments.
Edward G. Shelley is currently manager
of the Space Sciences Laboratory at the
Lockheed Palo Alto Research Laboratory.
Federico Herrero received the BS degree
from Spring Hill College in 1965 and the
MS and PhD degrees from the University
of Florida in 1966 and 1968, respectively,
all in physics. Following postdoctoral work
at Johns Hopkins University, he joined the
He has been involved in experimental
space physics research for more than 25
years, primarily in the area of solar-
./
terrestrial physics. He has been a principal
investigator or co-investigator on 14
NASA, ESA, and U.S. Department of Defense space research missions and is cur-
rently a member of seven mission or
mission-definition teams. He has spent his entire professional career
at the Lockheed Palo Alto Research Laboratory. He received his
PhD in nuclear physics from Stanford University in 1967. He received an MS in physics from Stanford in 1962 and BS degrees in
physics and mathematics from Oregon State University in 1959.
spent two years at the Weizmann Institute
of Science in Rehovot, Israel, investigating
faculty at the University of Puerto Rico
where he held the positions of professor
and chairman of the Physics Department
and performed thermospheric physics research at the National Astronomy and Ionosphere Center, Arecibo Observatory. In 1981, he joined the scientific staff of the NASNGoddard Space Flight Center. His current
research interests are in upper-atmospheric and magnetospheric
physics.
OPTICAL ENGINEERING I February 1 994 I Vol. 33 No. 2 I 369
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GHIELMETTI et al.
Mark F. Smith is a space physicist at the
Goddard Space Flight Center (GSFC),
Maryland. His major areas of research are
in the magnetopause, magnetosphericionospheric coupling, and space plasma
instrumentation. He received a BSc (hon-
ors) in physics and astronomy in 1980
from University College London and a PhD
in upper-atmospheric physics in 1984. He
was a research assistant at Mullard Space
Science Laboratory, United Kingdom, from
1984 to 1987. From 1987 to 1990, he was a senior research scientist
at the Southwest Research Institute. He joined the Laboratory of
Extraterrestrial Physics at NASA/GSFC in 1991. Smith has published more than 60 papers in various professional journals. He is a
member of the American Geophysical Union.
Thomas S. Stephen: Biography and photograph not available.
370 / OPTICAL ENGINEERING / February 1994/Vol. 33 No. 2
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